1. Field of the Invention
This invention relates to a signal analyzer and, more particularly, to a sweep measuring type signal analyzer such as a spectrum analyzer (including an optical spectrum analyzer), a network analyzer, and the like, which has a partially scrolling function for a screen that displays a measurement result.
Further, this invention relates to a signal analyzer such as a spectrum analyzer, used for spectrum analysis of a signal to be measured and, more particularly, to a signal analyzer with which an operator can easily obtain a display of data at a frequency outside a currently displayed frequency range in a visible format.
2. Description of the Related Art
A conventional spectrum analyzer normally employs a two-dimensional coordinate system wherein a frequency is plotted along the abscissa and a level of an analyzed signal is plotted along the ordinate. In this case, in order to adjust the frequency axis at a desired position, or in order to sequentially scan and observe data on the left or right side of a current display, data along the frequency axis is sometimes shifted in either direction. In general, the sequential shifting of data on the display screen is called a scrolling operation names after development of a "scroll". This ivention relates to a spectrum analyzer having a scrolling function which is easy to operate for an operator.
A typical scrolling function of a conventional display is (1) a scrolling function of a computer graphic display for shifting the display content on a display screen in the vertical direction in units of characters or dots. Another typical scrolling function is (2) a function of changing display data by half a display screen as a function similar to the scrolling function. The operation is as shown in FIGS. 1A and 1B. More specifically, FIG. 1A shows a left-shift operation for shifting an image toward a lower frequency along the frequency axis. FIG. 1B shows a right-shift operation for shifting an image toward a higher frequency along the frequency axis. In either case, an image (indicated by dotted lines a, b, and c) before a half-screen shift command is input is subjected to sweeping from d after the half-screen shift command is input. In this case, data are sequentially updated from the left end of the sweeping (in the order defined by d, e, and f), and an image indicated by a solid line is obtained. Therefore, initial data still remain on an unswept portion (portion of b-c when portion of a is swept, i.e., a portion wherein no data is updated). After sweeping is completed (upon completion of half-screen shifting), desired data (indicated by a solid line defined by d, e, and f) appears on the screen.
The related arts (1) and (2) respectively have the following problems. In the related art (1), the scrolling function of the computer graphic display has different properties from those of a scrolling function of a spectrum analyzer in which an unknown signal is measured while being swept and a measurement result must be displayed, and this function cannot be directly used.
The half-screen shift function of the related art (2) has the following two problems. (i) A frequency sweep rate has a physical limitation determined by a bandwidth inside a spectrum analyzer, and cannot be infinitely increased. Therefore, a portion to be observed cannot be reached faster than the sweep rate due to the above limitation. In other words, if the sweep rate is low, measurement cannot be immediately performed. In the case of FIG. 1A, a right-half screen is to be observed when the left-shift operation is performed. During the sweep time of the left-half screen, the right-half screen to be observed does not appear. That is, the left-half screen is an unnecessary portion for the operator. In the case of FIG. 1B, when data adjacent to the left end of the screen is to be observed by a right-shift operation, a desired screen does not appear until almost all the left-half screen sweep operation is completed. (ii) Data before and after scrolling cannot be continuously observed. In particular, in the case of the right-shift operation of FIG. 1B, although new data is to be observed as data continuous with the data before scrolling in the left direction, new data appears from the left end to the right. More specifically, a direction to be observed along the frequency axis, i.e., sweeping (from the right to the left) opposite to normal left-to-right sweeping cannot be performed, resulting in inconvenience.